Trematode parasites in snail hosts have been proposed as potential bioindicators of estuarine ecosystem health. Both the definitive and snail host must be present in the system for successful transmission and the diversity and abundance of larval trematodes in snail first intermediate host populations directly reflects the diversity and abundance of definitive hosts in the ecosystem. In addition, trematodes in snail hosts are negatively influenced by environmental impacts and abundance and diversity are lower in impacted habitats. Estuarine snails proposed as suitable bioindicators include species from the Potamididae, Cerithiidae, Nassariidae, Batillariidae and Hydrobiidae. The northern hemisphere snail Nassarius obsoletus hosts nine species of parasite from eight families that infect fish, birds or reptiles. Very little is known about parasites of snails in Australia estuaries. A study of three estuarine locations in Capricornia compared the parasites of three species of Nassarius: N. dorsatus, N. olivaceus and N. pullus. Trematodes from six families that infect fish and birds were collected, including eight species from a single family of fish parasites. Fish parasites were present across all locations, but bird parasites were collected from a single location. This location is listed as an important bird habitat, both for endangered and migratory species. No reptile parasites were collected from these locations, although a reptile parasite was recently reported from N. dorsatus in Townsville. In addition to indicating ecosystem health, parasites in snail hosts may indicate the relative importance of selected locations for particular types of definitive host species; especially birds, reptiles or mammals. Using parasite information as an additional bioindicator for estuarine health may help to inform the presence of important definitive hosts.